<?php
class system_db_expr {
	/**
	 * Storage for the SQL expression.
	 *
	 * @var string
	 */
	protected $_expression;
	
	/**
	 * Instantiate an expression, which is just a string stored as
	 * an instance member variable.
	 *
	 * @param string $expression The string containing a SQL expression.
	 */
	public function __construct($expression) {
		$this->_expression = (string) $expression;
	}
	
	/**
	 * @return string The string of the SQL expression stored in this object.
	 */
	public function __toString() {
		return $this->_expression;
	}

}